This invention relates to apparatus for producing duplex copies. More particularly, this invention relates to apparatus for transferring unfixed transferable images to opposite sides of a succession of copy sheets.
Several techniques are known in the prior art for forming duplex images on a final copy medium such as a web or sheet. One such technique requires the use of two photoconductors upon which first and second transferable images are formed respectively. The copy medium is generally passed between the photoconductors and the first and second images are transferred to opposite sides of the copy medium. Another technique similar to the above but involving the use of only one photoconductor, utilizes an intermediate image storage member such as a drum, to receive the first image formed on the photoconductor before transfer to a final copy medium. These techniques suffer the disadvantages of increased cost, machine complexity and size and decreased reliability necessitated by the use either of two photoconductive and optical systems or of additional components before transfer to a final copy medium.
A further duplexing technique utilized in certain commercial electrophotographic machines utilizes a "two-pass" process wherein copies developed sequentially on the first sides of a plurality of copy sheets by an electrographic process are collected in an intermediate tray and fed seriatim back through the process to develop second fixed images on the second sides of the copy sheets, thus producing duplex copies. This process is disadvantageous for several reasons. First, a duplex copy is not available for inspection until all of the first sides of the copy sheets and one set of the second sides thereof have been developed. In addition, the relatively long paper paths required in passing a copy sheet through the electrophotographic process twice greatly increases the possibility of paper jams and other potential copy handling complications. Moreover, environmental conditions of image formation and the physical parameters of the copy sheet may change resulting in images of varying quality on opposite sides of a single sheet and misregistration between images on opposite sides of a copy sheet may also result.
Another duplex copying technique involves fixing images to both sides of a copy sheet having a single pass through the disclosed electrophotographic processes. U.S. Pat. No. 3,506,347; 3,672,765; 3,869,202; and 3,947,270 disclose various embodiments of this technique. In the patent just listed a first tackified image is formed on a transfer drum, the image is transferred to the first side of a copy sheet, and the sheet is inverted while the first tackified image dries and becomes fixed on the copy sheet. Thereafter, a second tackified copy image is formed on the transfer drum and the copy sheet is fed back into contact with the drum to transfer the second image to the second side of the copy sheet which is then transported to an output tray. In the latter three patents, electrophotographic apparatus is disclosed for making copies wherein two images of an original are formed sequentially on a photoconductor, the images are developed and the first developed image is transferred to the first side of a copy sheet. The copy sheet is passed through a fuser to fuse or fix the first transferred image, is turned over and the opposite side of the copy sheet is brought into contact with the second developed image on the photoconductor. The second image is then transferred to the second side of the copy sheet, the copy sheet separated from the photoconductor and the second image fused by means of a second fuser. The disclosed techniques have several disadvantages. Since the first image is fixed before transfer of the second image, either two fusers must be used with attendant increase is cost, power and environmental heat or solvent fixing is used resulting in safety hazards and the inconvenience of handling consumable liquids.
U.S. Pat. No. 4,035,073 discloses a reproduction apparatus wherein first and second toner images are alternately formed on the first sides of a series of three copy sheets, the first sides of the first and third sheets receiving the first image and the first side of the second sheet receiving the second image. The copy sheets are then fed along a conveyer belt with the side carrying the toner image contacted first by a lifter mechanism and then by a conveyor belt. The three sheets are then brought back into contact with the drum so that the second sides of the first and third sheets receive the second images and the second sheet receives the first image. This apparatus is disadvantageous in that three sheets must be processed before a duplex copy is ready for inspection. Furthermore, the long copy sheet paths and long times between first and second image transfer to a copy sheet increases the likelihood of image quality variations on opposite sides of a sheet due to changes in environmental conditions. In addition, image blurring and degradation is greatly increased through contacting the imaged side during copy sheet inversion by the lifter and belt.
In U.S. Pat. No. 4,095,979, issued June 20, 1978, there is disclosed a single pass duplexing technique utilizing a reversible vacuum belt for inverting a copy sheet to the first side of which an unfixed image has been transferred by contacting the unimaged second side of the sheet.
A further disadvantage of many of the known duplexing techniques is the inability to handle heavy copy sheet stock. Most of these techniques are designed to handle copy sheet stock which is quite flexible and capable of being transported through paths having relatively sharp turns. Heavier paper and card stock may be relatively inflexible and incapable of negotiating such sharp sheet paths.